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Soil/Transcript
Transcript A boy, Tim, is planting seeds in the soil. A robot, Moby, watches him. Moby pours water from his fingertip. Then his fingertip lifts up and a rolled up letter falls out. MOBY: Beep. Tim reads from the typed letter. TIM: Dear Tim and Moby, Where does soil come from? From, Juno. Soil is a blend of rock and mineral fragments, water, air, and humus. That's carbon-rich, or organic, material left behind by living things. An image shows a pile of rocks, water, air, and humus. MOBY: Beep. TIM: It all starts with the rocks and minerals that form the earth's crust. An image shows the earth with a section removed, revealing its inner layers. The earth’s crust is its outer edge. TIM: Things like wind, water, ice, carbon dioxide, and oxygen break the crust down in two processes called weathering and erosion. Animations and images show blowing trees, rain falling, ice, and molecules labeled CO2 and O2. TIM: Weathering is the process of breaking down rocks, soil, and minerals through contact with the atmosphere. An animation shows rain falling on a rocky surface. TIM: Heat, pressure, water, and other chemicals can cause a rock to become weathered. Images show fire, a rock with several arrows pointing at it indicating pressure, water, and a green liquid chemical with vapor rising from it. TIM: And erosion is something that causes rocks to move and disintegrate by the action of water, wind, gravity, and ice. Images show a river and ice. Animations show trees blowing in the wind, an arrow moving down toward the ground indicating gravity, and a chunk of ice. TIM: As a rock erodes and weathers over many years, it breaks down into tiny pieces, and small plants are able to grow in it. The roots of the plants break the rock down even more. Humus made of dead plants accumulates in the rock. The humus makes the dirt more livable, not only for more complex plants, but also for animals that live in the ground, like worms and insects. As more and more plants and animals live in the soil, its humus content rises, and it becomes more and more fertile. The animation of rain falling on a rocky surface is shown again. The rain stops and plants sprout. Their roots grow down and break apart the rocks. The plants die and turn into humus. More rain falls. The rocky surface turns brown, larger plants grow, and animals and insects appear. MOBY: Beep. TIM: The process can take thousands and thousands of years. A split screen shows a rocky gray surface on one side, and a frog, worm, plants, and brown soil on the other. TIM: But if you've ever dug a deep hole, you can actually see the different stages of soil in the process of forming. Moby has a shovel and starts digging a hole in the backyard. TIM: Soil is made up of three layers, or horizons. Horizon A, or topsoil, contains lots of humus, which makes it the darkest layer. The A horizon is covered by a layer of litter, made up of leaves, twigs, and other organic material. Moby digs a deep hole on both sides of Tim. The three layers are revealed. Moby points to horizon A, the darkest layer or topsoil. TIM: This stuff will eventually decompose into more humus. Horizon B, or subsoil, contains weathered rocks and minerals and clay. It has much less humus than the A horizon, so it's lighter in color and much less fertile. Horizon B contains some organic material washed down from the A horizon through leaching. Tim scoops up some dirt in his right hand and holds humus in his left hand. Moby points to horizon B. It is a lighter color than horizon A. MOBY: Beep. TIM: When water soaks into the topsoil, it reacts with the humus to form an acid. This acid dissolves some of the nutrients in the A horizon and drips down into the B horizon, making it a bit more fertile. The same thing happens in horizon C, which is made up of partially weathered rocks and minerals, as well as leached material from the B horizon. Together the three horizons form the soil profile. An animation shows water reaching the three horizons of soil. Rain goes through the soil forming acid that drips through the A, B, and C horizons. Another image shows Moby standing next to the soil profile: horizons A, B, and C. MOBY: Beep. TIM: Below that? Well, if you keep digging, you'll hit rock that hasn't been broken down yet: bedrock. An image shows horizons A, B, and C and some gray rock below that, called "bedrock." Moby continues to dig and breaks his shovel. TIM: I warned you. MOBY: Beep. Category:BrainPOP Science Transcripts Category:BrainPOP Transcripts